Composite photoanodes

ABSTRACT

The provided method includes photoelectrodeposition of an electrocatalyst onto a semiconductor to form a photoanode. The method yields composite photoanodes showing enhancement of photocurrent (water splitting rate) when incorporated into a photoelectrochemical cell for water electrolysis.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of International Application No. PCT/US2011/027603, filed Mar. 8, 2011, which claims the benefit of U.S. Provisional Application No. 61/311,724, filed Mar. 8, 2010; the disclosure of each application is incorporated herein by reference in its entirety.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

This invention was made with Government support under the Integrative Graduate Education and Research Traineeship (IGERT) awarded by the National Science Foundation (Award No. DGE-050-4573). The Government has certain rights in the invention.

BACKGROUND

The photoelectrochemical (PEC) conversion of photon power into chemical fuels offers an attractive approach to storing solar energy, but it poses many fundamental chemical challenges. Hematite (α-Fe₂O₃) has emerged as a prototype photoanode material for testing strategies to overcome the challenging 4-electron oxidation of water, which under basic conditions is described by Equation 1:

4OH⁻→O₂+4e ⁻+2H₂O  (1)

Hematite meets many of the target photoanode requirements: It is inexpensive, oxidatively robust, environmentally benign, and it absorbs visible light (E_(g)˜2.1 eV). Although the α-Fe₂O₃ valence band edge potential is about 1 V or higher more positive than required for Equation 1 thermodynamically, water oxidation by photogenerated valence-band holes in α-Fe₂O₃ is kinetically inefficient, and additional anodic overpotentials are typically required before significant PEC water splitting is observed. A remaining fundamental limitation of α-Fe₂O₃ is that its conduction band edge potential resides ˜200 mV below that required to drive the cathodic half reaction (Equation 2).

2H₂O+2e ⁻→H₂+2OH⁻  (2)

Tandem PEC/photovoltaic (PV) configurations have been envisioned to provide the bias needed to meet these demands. Recent advances in controlled growth and doping of α-Fe₂O₃ nanostructures attempt to overcome many of the limitations associated with the short hole-diffusion length (˜2-4 nm), low electron mobility (˜10⁻¹ cm² V⁻¹ s⁻¹), and efficient charge carrier recombination characteristics of bulk α-Fe₂O₃ yielding promising PEC performance. For example, an overall solar-to-hydrogen power conversion efficiency of ˜2.1% has been estimated for one set of mesostructured α-Fe₂O₃ photoanodes when powered by a PV device providing 1.4 V in a tandem configuration. Unfortunately, many low-cost PV devices such as dye-sensitized solar cells or organic PVs typically provide about 1 V or lower, and two such PVs in series would thus be required to provide the necessary 1.4 V. The development of α-Fe₂O₃ photoanodes that require smaller overpotentials to oxidize water, such that they could be powered by single low-cost PV cells, would thus be attractive for reducing solar hydrogen production costs.

Recently, electrochemical water oxidation with low overpotentials was demonstrated over a range of pH values using an amorphous cobalt/phosphate catalyst (“Co-Pi”) electrodeposited onto ITO or FTO electrodes. Remaining uncertainties about the catalyst's precise microscopic identity do not diminish its attractiveness for water-splitting PECs. Co-Pi requires 0.41 V overpotential at pH 7 to oxidize water with a current density of 1 mA/cm², whereas the α-Fe₂O₃ valence band edge potential provides about 1.2 V or higher. Photogenerated holes in α-Fe₂O₃ should thus be amply capable of driving water oxidation by this electrocatalyst.

What is desired, therefore, are new methods and materials for forming improved photoelectrodes for use in PECs.

SUMMARY

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

Composite photoanodes and methods for making the composite photoanodes are provided. The composite photoanodes comprise a semiconductor and an electrocatalyst.

In one aspect, a method of forming a composite photoanode by photoelectrodeposition is provided. In one embodiment, the method comprises photoelectrodepositing a solid conformal layer of an electrocatalyst from an electrolyte solution on a surface of a semiconductor submerged in the electrolyte solution by simultaneously:

-   -   (1) impinging the surface of the semiconductor with         electromagnetic radiation having a first wavelength and a first         irradiance, to provide a first photoenergy that is sufficient to         excite an electronic transition of the semiconductor; and     -   (2) applying a first electric bias to the semiconductor, wherein         the first electric bias is less than an electrochemical         deposition bias, said electrochemical deposition bias being the         minimum voltage required to electrodeposit the electrocatalyst         onto the surface of the semiconductor without impinging the         surface of the semiconductor with electromagnetic radiation         having the first photoenergy.

In another aspect, a method for making an electrode is provided. In one embodiment, the electrode is formed by deposition of a competent electrocatalyst onto a photoanode from an electrolyte, wherein the deposition can be carried out by photodeposition, electrochemical deposition or a combination thereof. The electrolyte may comprise inorganic and organic ions, such as phosphate anion, acetate anion, sulfate anion, chloride, nitrate, sodium, potassium, or any combination thereof.

In another aspect, an electrode is provided comprising:

a photoanode having a first onset potential when incorporated into a photoelectrochemical cell for electrolysis of water into oxygen; and

a layer of an electrocatalyst conformally formed on a surface of the photoanode, wherein said electrocatalyst causes a cathodic shift in an onset potential of the electrode such that the electrode has a second onset potential when incorporated into a photoelectrochemical cell for electrolysis of water into oxygen, and wherein said second onset potential is less than said first onset potential.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIGS. 1A-1D. SEM images of mesostructured α-Fe₂O₃ photoanode before (FIGS. 1A, 1B), and after (FIGS. 1C, 1D) 1 hour electrochemical deposition of Co-Pi catalyst. The catalyst layer cracking occurs upon drying for the SEM measurement and in some cases allows inspection of the catalyst underside: FIG. 1D shows that the Co-Pi underside topology conforms to the α-Fe₂O₃ mesostructure.

FIGS. 2A-2C. (FIG. 2A) Dark (dashed) and photocurrent (solid) densities for α-Fe₂O₃ and Co-Pi/α-Fe₂O₃ photoanodes collected using simulated AM1.5 illumination (1 sun, back-side) at a scan rate of 50 mV/s. (FIG. 2B) Electronic absorption and (FIG. 2C) IPCE spectra for α-Fe₂O₃ and Co-Pi/α-Fe₂O₃ (at 1.23 and 1 V vs RHE, respectively). The absorption spectrum of Co-Pi on FTO without α-Fe₂O₃ is included in FIG. 2B, but no photocurrent was detected for these anodes.

FIG. 3. Dark (dashed) and photocurrent (solid) densities for mesoscopic α-Fe₂O₃ photoanodes used in this study under back-side and front-side illumination, collected using simulated AM1.5 sunlight (1 sun). Scan rate 50 mV/s.

FIG. 4. Absorption spectrum of Co(OH)₄ ²⁻ at pH ˜13 measured ˜30 min after preparation of the solution.

FIG. 5. Summary of solar water-splitting PECs using Co-Pi/α-Fe₂O₃ composite photoanodes and Pt counter electrode.

FIG. 6. Co-Pi electrochemical deposition on α-Fe₂O₃ photoanode in pH 7, 0.1 M KPi buffer at +1.1 V vs Ag/AgCl for 30 min.

FIG. 7. Co-Pi electrochemical deposition on α-Fe₂O₃ photoanode in pH 7, 0.1 M KPi buffer at +1.1 V vs Ag/AgCl for 15 min.

FIG. 8. Photocurrent decay of α-Fe₂O₃ photoanode measured in pH 7, 0.1 M KPi buffer at +1.3 V vs RHE, 1 sun AM 1.5.

FIGS. 9A and 9B. Dark current (dotted) and photocurrent (solid and dashed) densities of α-Fe₂O₃ photoanodes before and after 30 min of Co-Pi deposition, measured in pH 13.6 NaOH (FIG. 9A), and pH 8 KPi (FIG. 9B) at 50 mV/s (thick line) and 10 mV/s (dashed line). The α-Fe₂O₃ data were collected at 10 mV/s. The circles denote steady state photocurrent densities after 200 s of continuous illumination under 1 sun, AM 1.5 simulated sunlight.

FIGS. 10A and 10B. (FIG. 10A) Dark current (dotted) and photocurrent (solid) densities of an α-Fe₂O₃ photoanode before (thin black) and after 30 min of Co-Pi deposition (thick lines) in 0.1 M KPi electrolyte at pH 8. The α-Fe₂O₃ data (black curves) were collected at 10 mV/s. Inset: Photocurrent density vs time at +1.1 V vs RHE. (FIG. 10B) Power dependence of photocurrent density for an α-Fe₂O₃ photoanode before (x) and after (+) Co-Pi deposition, measured at +1.0 V vs RHE.

FIG. 11. O₂ generation and photocurrent density over time measured for a Co-Pi/α-Fe₂O₃ composite photoanode at +1.0 V vs RHE in 0.1 M KPi at pH 8. Co-Pi was electrodeposited on α-Fe₂O₃ for 30 min. The top panel showed an initial spike in the rate of O₂ evolution before relaxation to a steady state rate.

FIG. 12. Linear sweep voltammetry of Co-Pi on FTO at various scan rates in pH 7, 0.1 M KPi electrolyte. Inset: Decay of the bulk electrolysis current density over time under these conditions, measured at +1.1 V vs Ag/AgCl (+1.3 V vs NHE).

FIGS. 13A-13D. Scanning electron micrographs of Co-Pi/α-Fe₂O₃ composite photoanodes after 15 minutes of Co-Pi deposition showing (FIG. 13A) ring-like deposition of Co-Pi in selective areas of the α-Fe₂O₃ surface, and (FIGS. 13B-13D) magnified views of Co-Pi patches within this ring. FIG. 13D showed Co-Pi conforming to the topology of the underlying α-Fe₂O₃ mesostructure. The cracks in the Co-Pi result from drying.

FIGS. 14A and 14B. Dark current (dotted) and photocurrent (solid) densities of an α-Fe₂O₃ photoanode before and after 15 min of Co-Pi deposition, measured in (FIG. 14A) pH 13.6 NaOH, and (FIG. 14B) pH 8 KPi with (black) and without 0.1 M NaCl at 50 mV/s (thin line) and 10 mV/s (thick line). The α-Fe₂O₃ data were collected at 10 mV/s. The circles denote steady state photocurrent densities after 200 s of continuous illumination under 1 sun, AM 1.5 simulated sunlight.

FIGS. 15A and 15B. Photocurrent decay curves measured under 1 sun, AM 1.5 simulated sunlight at various applied potentials for Co-Pi/α-Fe₂O₃ composite photoanodes in pH 8 KPi, pH 8 buffered salt water, and pH 13.6 NaOH (black) electrolytes. (FIG. 15A) Data collected following 30 min of Co-Pi deposition, and (FIG. 15B) data collected following 15 min of Co-Pi deposition. Photocurrent decay curves measured for α-Fe₂O₃ in pH 8, 0.1 M KPi electrolyte (grey) are included in FIG. 15B for comparison.

FIGS. 16A-16D. SEM images of Co-Pi/α-Fe₂O₃ composite photoanode after 30 min of Co-Pi deposition.

FIGS. 17A and 17B. (FIG. 17A) Dark current (dotted) and photocurrent (solid) densities measured for an α-Fe₂O₃ photoanode before and after 15 min of Co-Pi deposition. Data collected at 10 mV/s, for both front- and back-side illumination. (FIG. 17B) Photocurrent density and O₂ generation measured for the above photoanodes vs time: Co-Pi/α-Fe₂O₃ (black) and α-Fe₂O₃ (grey). The numbers in (FIG. 17B) indicate the photocurrent and O₂ enhancement factors (see text). Bubbles adhering to and releasing from the photoanode surface cause disruptions in the current density. All PEC data were collected under 1 sun, AM 1.5 simulated solar irradiation.

FIG. 18. Dark current (dotted) and photocurrent (solid) densities measured for an α-Fe₂O₃ photoanode before and after Co-Pi photoelectrochemical deposition. Data was collected at 10 mV/s with front-side illumination under 1 sun AM 1.5 simulated sunlight. Co-Pi was deposited for 10 min at ˜100 μA/cm².

FIG. 19. Dark current (dotted) and photocurrent (solid) densities measured for an α-Fe₂O₃ photoanode before and after CoO_(x) electrochemical deposition. Data was collected at 10 mV/s with front-side illumination under 1 sun AM 1.5 simulated sunlight. CoO_(x) was deposited for 15 min at 50 μA/cm² and ˜1.1-1.3 V vs Ag/AgCl.

FIG. 20. Dark current (dotted) and photocurrent (solid) densities measured for an α-Fe₂O₃ photoanode before and after CoO_(x) photoelectrochemical deposition. Data was collected at 10 mV/s with front-side illumination under 1 sun AM 1.5 simulated sunlight. CoO_(x) was deposited for 180 sec at 10 μA/cm² and ˜0.1-0.3 V vs Ag/AgCl.

FIGS. 21A-21C illustrate three potential methods for adsorbing cobalt to a photoanode.

FIGS. 22A-22H. Wide- and narrow-angle SEM images of an unmodified α-Fe₂O₃ mesostructured photoanode, and similar α-Fe₂O₃ photoanodes following Co-Pi electrochemical deposition, photo-assisted electrochemical deposition of Co-Pi, and Co²⁺ adsorption.

FIGS. 23A-23C illustrate ark-current (dotted) and photocurrent (solid) densities of (23A) a Co-Pi/α-Fe₂O₃ electrode prepared by photo-assisted electrochemical deposition, (23B) a Co-Pi/α-Fe₂O₃ electrode prepared by electrochemical deposition, and (23C) a Co²⁺/α-Fe₂O₃ electrode prepared by surface adsorption, compared to the parent α-Fe₂O₃ photoanodes.

FIGS. 24A-24C illustrate dark-current (dotted) and photocurrent (solid) densities of (24A) a Co-Pi/α-Fe₂O₃ electrode prepared by photo-assisted electrochemical deposition, (24B) a Co-Pi/α-Fe₂O₃ electrode prepared by electrochemical deposition, and (24C) a Co²⁺/α-Fe₂O₃ electrode prepared by surface adsorption, compared to the parent α-Fe₂O₃ photoanodes.

FIGS. 25A-25C illustrate the best (filled bars) and average (empty line) cathodic shifts, photocurrent density increases, and onset potentials for Co-Pi/α-Fe₂O₃ photoanodes prepared by photo-assisted electrochemical deposition (P-Dep) and electrochemical deposition (E-Dep) of Co-Pi, and for Co²⁺/α-Fe₂O₃ photoanodes prepared by surface adsorption of Co²⁺(Co-dip).

FIG. 26 illustrates the average cathodic shifts plotted vs average onset potentials for Co-Pi/α-Fe2O3 photoanodes prepared by photo-assisted electrochemical deposition (P-Dep) and electrochemical deposition (E-dep) of Co-Pi, and for Co2+/α-Fe2O3 photoanodes prepared by surface adsorption of Co2+ (Co-dip) for the 12 films of FIG. 5. The open symbols represent the parent α-Fe2O3 photoanodes. These data show a strong correlation between the two performance metrics, with photo-assisted electrochemical deposition of Co-Pi leading to the lowest onset potentials and the greatest cathodic shifts.

FIG. 27 illustrates the Average photocurrent density increase vs photocurrent at 1.1 V vs RHE (one-sun) for the 12 films of FIG. 5. Photo-assisted electrochemical deposition of Co-Pi yields the largest photocurrent density increases and the highest absolute photocurrent densities. The open symbols (grouped at the base of the dashed line) represent the parent α-Fe2O3 photoanodes.

FIG. 28 illustrates the time dependence of the photocurrent density of a Co-Pi/α-Fe2O3 photoanode prepared by photo-assisted electrochemical deposition, measured at 1.0 V vs RHE in 1 M NaOH under continuous 1 sun, AM 1.5 simulated solar irradiation. The electrolyte was not stirred. The electrolyte was replaced after 75 hrs (dashed line), resulting in recovery of photocurrent density.

FIG. 29 illustrates current density-voltage curves of a TiO₂ nanowire photoanode before and after Co-Pi photoelectrochemical deposition, under 1 sun, AM 1.5 simulated solar irradiation (solid) and in the dark (dashed) in 0.1M potassium phosphate buffer at pH 7.

FIG. 30 illustrates current density-voltage curves of a TiO₂ nanowire photoanode sensitized with CdS and coated with a thin amorphous TiO₂ protective layer, before and after Co-Pi photoelectrochemical deposition, under 1 sun, AM 1.5 simulated solar irradiation in 0.5M sodium thiosulfate.

FIG. 31 illustrates current density-voltage curves of a Co²⁺:Zno photoanode before and after Co-Pi photoelectrochemical deposition, under 1 sun, AM 1.5 simulated solar irradiation (solid) and in the dark (dotted) in 0.1M potassium phosphate buffer at pH 11.

FIG. 32 illustrates current density-voltage curves of a W doped BiVO₄ photoanode before and after Co-Pi photoelectrochemical deposition, under 1 sun, AM 1.5 simulated solar irradiation (solid) and in the dark (dashed).

FIG. 33 illustrates current density-voltage curves of an α-Fe₂O₃ photoanode before and after cobalt methyl-phosphonate (Co-MePi) photo-assisted electrochemical deposition, under 1 sun, AM 1.5 simulated solar irradiation (solid) and in the dark (dotted) in 1 M NaOH, pH 13.6.

FIG. 34 illustrates current density-voltage curves of an α-Fe₂O₃ photoanode before and after nickel borate (Ni—Bi) electrochemical deposition, under back-side illumination with 1 sun, AM 1.5 simulated solar irradiation (solid) and in the dark (dotted) in 1 M NaOH, pH 13.6.

DETAILED DESCRIPTION

The composite photoanodes provide enhanced performance compared to known photoanodes when incorporated into photoelectrochemical (PEC) systems (e.g., a PEC cell for splitting water into hydrogen). One particular benefit of the composite photoanodes is a cathodic shift in the onset potential of the photoanode when used in a PEC system. Such a cathodic shift allows for reduced electrical requirements to drive the PEC process, thereby increasing the efficiency of such systems.

The methods for forming composite photoanodes are light-enhanced deposition methods, referred to herein as photoelectrochemical deposition methods. Photoelectrochemical deposition of an electrocatalyst onto a semiconductor to form a composite photoanode provides enhanced photoanode performance in PEC systems, including increased cathodic shift, compared to composite photoanodes fabricated using traditional electrochemical methods. Such improvements are attributed to the conformal nature of electrocatalyst layers formed using photoelectrodeposition.

In one aspect, a composite photoanode is provided, said composite photoanode comprising a semiconductor having a solid conformal layer of an electrocatalyst formed on its surface. Herein, the electrocatalysts referred to are competent electrocatalyst that produce a cathodic shift in the onset potential of the photoanode when used within a PEC.

The semiconductor acts as a photoanode. The semiconductor is made of a photon-absorbing material. In one embodiment, the semiconductor is α-Fe₂O₃ (“hematite”). In another embodiment, the α-Fe₂O₃ semiconductor is mesostructured. In another embodiment, the semiconductor is a high-surface-area α-Fe₂O₃ photoanode. The use of hematite as a semiconductor is disclosed extensively herein, including in Examples 1-4 and 10.

While hematite is a preferred semiconductor, other semiconductors are contemplated. For example, Examples 5 and 6 disclose the use of titanium dioxide (particularly in nanowire form) as a semiconductor; Example 7 discloses the use of a cobalt-ion:zinc oxide semiconductor; and Example 8 discloses the use of a W:BiVO4 semiconductor. These listed examples are generally inorganic in character.

In one embodiment, the semiconductor comprises a group IV semiconductor having a formula selected from the group consisting of binary, ternary, and quaternary. In a further embodiment, the group IV semiconductor further comprises ions selected from the group consisting of cations and anions.

In one embodiment, the semiconductor is an n-type semiconductor.

Other photoanode materials useful in composite electrodes include a material selected from the group consisting of an iron oxide, a zinc oxide, a titanium oxide, a tungsten-bismuth-vanadium oxide, a tungsten oxide, a gallium-zinc-oxide-nitride, or these materials also containing additional cations or anions.

In another embodiment, the semiconductor comprises a sensitizer having a sensitizer absorbance wavelength, said sensitizer absorbance being different from a semiconductor absorbance wavelength. By incorporating a sensitizer with the semiconductor (e.g., embedded within, or coating the surface of the semiconductor), a broader spectrum of light can be usefully absorbed by the semiconductor and overall composite photoanode for use in the PEC process. Representative sensitizers include cadmium selenium, as described below in Example 6. Additional sensitizers include cationic or anionic impurities

In one embodiment, the semiconductor has a physical shape selected from the group consisting of dendrites, wires, belts, rods, mesostructures, nanotubes, and thin films. As described further in Example 2, a high semiconductor surface area and/or a high composite photoanode surface area produces improved results for PEC reactions. Certain physical shapes, such as dendrites, etc. are known to create a relatively high surface area. Such physical shapes are preferred in the embodiments provided herein. Nanoscopic high-surface area shapes are accordingly preferred. Therefore, in another embodiment, the physical shape has nanoscopic dimensions. As used herein, the term nanoscopic dimensions refers to a shape having at least one feature (e.g., dendrites) having a smallest size of 100 nm or smaller.

Semiconductors can be deposited on substrates, or otherwise formed, according to methods known to those of skill in the art, including those provided below in the Examples. For example, hematite can be grown using chemical vapor deposition (see Example 1).

The electrocatalyst is formed on a surface (e.g., a surface that will face a light source during PEC) of the semiconductor. In certain embodiments, the electrocatalyst produces a cathodic shift in the onset potential of a PEC process incorporating a composite electrode (semiconductor and electrocatalyst) when compared to the semiconductor alone. This comparison of electrodes can be found throughout the data provided herein so as to illustrate the efficacy of the disclosed materials, devices, and methods, in improving the PEC performance of semiconductors.

In one embodiment, the electrocatalyst is selected from the group consisting of a cobalt-containing catalyst, an iridium-containing catalyst, a manganese-containing catalyst, a ruthenium-containing catalyst, a nickel-containing catalyst (e.g., nickel borate, see Example 10), a cobalt-containing oxygen evolving catalyst, a cobalt oxide/hydroxide catalyst, and a cobalt oxide catalyst.

In one embodiment, the electrocatalyst is cobalt phosphate (Co-Pi). Co-Pi is used extensively in the examples provided herein. Examples 1-3 and 5-9 describe the use of Co-Pi to improve PEC performance.

In one embodiment, the layer of the electrocatalyst has a thickness of from 0.5 nm to 30 nm. As will be described further below in Example 2, a thick electrocatalyst layer will inhibit performance of composite photoanodes. Accordingly, nanoscale-thick electrocatalyst films are preferred. Electrochemical deposition does not allow for quality films of such a thickness to be deposited. Accordingly, photoelectrodeposition is preferred for forming nanoscale-thick films of electrocatalyst.

Examples of the competent electrocatalyst include, without limitation, a cobalt catalyst, iridium catalyst (e.g. IrO₂), manganese catalyst (e.g. Mn-oxo complexes), ruthenium catalyst (e.g. [Ru(L)₂(OH)₂]²⁺ complexes, where L denotes ligand). In one embodiment, the cobalt catalyst was selected from the group consisting of cobalt based oxygen evolving catalyst and cobalt oxide catalyst (referred herein as “CoO_(x)”, see Example 4).

The photoelectrochemical performance of composite the photoanodes provided herein is improved compared to “semiconductor-only” photoanodes. The EXAMPLES describe these improvements extensively.

For example, Co-Pi/α-Fe₂O₃ composite photoanodes for water oxidation are improved by optimization for front-side illumination in pH 8 electrolytes. Without being limited by theory, it is believed that a kinetic bottleneck appears to be related to the Co-Pi catalyst itself under these conditions. This kinetic bottleneck is overcome by more sparse deposition of Co-Pi onto α-Fe₂O₃. Following these improvements, sustained water oxidation by Co-Pi/α-Fe₂O₃ composite photoanodes was demonstrated in both photocurrent and O₂ evolution measurements. Photoelectrochemical water oxidation by the Co-Pi/α-Fe₂O₃ composite photoanodes was enhanced relative to that of α-Fe₂O₃ alone: Under these conditions, a five-fold enhancement in the photocurrent density and water oxidation rate was observed at +1.0 V vs RHE. This enhancement is even more substantial at about 1.0 V or lower vs RHE, where α-Fe₂O₃ alone does not exhibit significant photocurrent at all.

It is also interesting to compare these results with those obtained for bulk electrolysis by Co-Pi without a photon-absorbing substrate. By itself, Co-Pi electrolysis current densities reached ˜1.2 mA/cm2 at an applied bias of +1.29 V vs NHE (pH 7), or ˜+1.7 V vs RHE. In conjunction with an inexpensive and robust photoanode such as α-Fe₂O₃ under 1 sun, AM 1.5 illumination, the applied bias necessary to achieve the same current density can be reduced by over 0.5 V in buffered salt water at pH 8, the average pH of sea water. The results described here thus demonstrate that sustained O₂ evolution in mild salt water conditions can be achieved with significantly reduced external power demands relative to Co-Pi alone, particularly in the low current density regime, by integrating this catalyst with a light-harvesting semiconductor substrate. The overall process, in which photogenerated holes in α-Fe₂O₃ are converted to oxidizing equivalents in Co-Pi, yielding O₂ evolution well below the Co-Pi bulk electrolysis threshold potential, is summarized schematically in FIG. 5. Further improvement of the performance of these composite photoanodes can be anticipated, for example, by variation of the Co-Pi deposition conditions to optimize photocurrent densities at extremely low bias. More generally, these results emphasize that composite photoanode strategies offer promising prospects for sustainable, affordable, and distributed solar fuel technologies. This is equally applicable to include other catalysts such as IrO₂, Mn-oxo complexes, or [Ru(L)₂(OH₂)]²⁺ complexes, that can be powered in part or entirely by light-harvesting electrodes.

The combination of the first photoenergy and the first electric bias are sufficient to deposit catalyst components from the electrolyte to form the solid conformal layer of the electrocatalyst.

The electrocatalyst is formed from a buffer solution in which the semiconductor is submerged. The electrolyte solution can be any electrolyte solution known to those of skill in the art. Particularly, electrolyte solutions useful for traditional electrochemical deposition of an electrocatalyst onto a semiconductor are useful in the method. In a preferred embodiment, the electrolyte solution is a buffer solution of potassium phosphate (e.g., at pH 7) containing Co(NO₃)₂ if Co-Pi is to be the formed electrocatalyst.

In one embodiment, the pH of the electrolyte is about 7 or higher. In one embodiment, the pH of the electrolyte was about 13 or higher. In another embodiment, the pH of the electrolyte was about 8.

The surface of the semiconductor is irradiated with electromagnetic radiation (“light”) having a first wavelength and a first irradiance. The light can be a single wavelength or a broadband source. The only requirement is that the light provides a photoenergy sufficient to produce an electronic excited state in the semiconductor so as to provide a portion of the energy required to deposit the electrocatalyst from the electrolyte solution. In one embodiment, the electronic transition is a bandgap transition. By exciting the bandgap transition, photogenerated valence band holes can oxidize ions in the electrolyte to from an active catalyst at the surface.

Because the first photoenergy is not sufficient to drive the deposition of the electrocatalyst from the electrolyte solution, a first electric bias is simultaneously applied to the semiconductor. The first electric bias is significantly less than the bias required for electrochemical deposition. In one embodiment, the first electric bias is from 0.1 V to 0.4 V (e.g., versus Ag/AgCl). Therefore, the deposition (i.e., the photoelectrochemical deposition) of the electrocatalyst on the semiconductor is accomplished in the method by using energy from two sources (light and electricity) to facilitate the deposition reaction from the electrolyte. Neither of the two energy sources alone is sufficient to facilitate the deposition on their own.

The method according to this aspect utilizes light (e.g., sunlight or artificial sunlight) to assist in electrochemical deposition of an electrocatalyst onto a semiconductor. The method is useful, for example, to fabricate a composite photoanode according to the other aspects and Examples provided herein.

Examples 1 and 2 below disclose composite photoanodes fabricated with the generally known technique of electrochemical deposition. These examples are contrasted, by further Examples 3-10 utilize photoelectrochemical deposition.

Example 3, below, provides an in-depth development of the theory and results of photoelectrodeposition. While Example 3 primarily describes composite photoanodes of Co-Pi and α-Fe₂O₃, the method is not limited to these compounds. As illustrated in other Examples, photoelectrochemical deposition is compatible with any known semiconductors and electrocatalysts, particularly those used to make photoanodes using electrochemical deposition.

Without being bound by theory, in principle, photogenerated holes can be used to oxidize an ion from an electrolyte. For example, with reference to Co-Pi deposition on hematite, Co²⁺ can be deposited to form Co-Pi on the α-Fe₂O₃ photoanode and the electron can be removed by water reduction. Because photogenerated electrons in the conduction band of α-Fe₂O₃ are below the energy needed to reduce protons to hydrogen, a very low bias is applied to assist in photoelectrochemical deposition. Hence the addition of “electro” to photoelectrochemical deposition. The bias required for photoelectrochemical deposition is lower than that required for electrochemical deposition of similar compounds (i.e., deposition without the assistance of light).

Any light source with sufficient energy to excite the band gap of the semiconductor can be used in photoelectrodeposition. For example, sunlight (or artificial sunlight) can be used to drive photoelectrochemical deposition so as to test the possibility of a sunlight-driven reaction. In an exemplary embodiment (described further in Example 3) photoelectrodeposition on α-Fe₂O₃ was conducted in a three-electrode configuration from a solution of Co²⁺ in potassium phosphate (KPi) buffer under 1 sun AM 1.5 simulated solar irradiation. A Pt mesh was used as the counter electrode and saturated Ag/AgCl was used as the reference electrode. Typical current densities during deposition were ˜1-100 μA/cm².

It will be appreciated that a broad-spectrum light source (e.g., sunlight) need not be used in the method, as any light source capable of exciting the bandgap of the semiconductor is compatible with the method. For example, a single wavelength light source can be sufficient to excite the bandgap as long as

One impetus for the development of photoelectrochemical deposition was to develop an electrocatalyst deposition method that would allow for nanoscale-thick, conformal, continuous layers (films) of electrocatalyst to be deposited on a semiconductor. Particularly if the semiconductor is nanostructured (e.g., dendritic). Traditional electrochemical deposition is insufficient in this regard. As demonstrated in the Examples (e.g., Example 3), thin, conformal electrocatalyst films are satisfactorily formed using photoelectrochemical deposition.

PEC reactions driven with photoanodes formed using photoelectrochemical deposition demonstrate improved absolute onset potential, cathodic shift of the onset potential, and maximum current density. All vital characteristics of, for example, PEC for water splitting, particularly in the context of solar-powered PEC devices.

In one embodiment, the electrocatalyst is selected from the group consisting of a cobalt-containing catalyst, an iridium-containing catalyst, a manganese-containing catalyst, a ruthenium-containing catalyst, a nickel-containing catalyst, a cobalt-containing oxygen evolving catalyst, a cobalt oxide/hydroxide catalyst, and a cobalt oxide catalyst.

In a preferred embodiment, the electrocatalyst is cobalt phosphate.

In one embodiment, the layer of the electrocatalyst has a thickness of from 0.5 nm to 30 nm.

In one embodiment, the cathodic shift is from 50 mV to 400 mV.

In one embodiment, the layer of the electrocatalyst has a thickness of from 0.5 nm to 30 nm. Such a thickness is indicative of the “thin” nature of the conformal electrocatalyst coating. As set forth herein, such a thin coating is essential, not only to allow light through the electrocatalyst to the semiconductor, but also due to the short charge diffusion lengths in the photoanode materials. Finally, as disclosed herein, thin films of electrocatalyst are less prone to defects (e.g., aggregates) than thicker electrocatalyst films are.

It will be appreciated that the range of 0.5 nm to 30 nm represents from about one molecular layer to about tens of molecular layers. Accordingly, it is preferred that a minimal number of molecular layers are used to conformally coat the semiconductor with electrocatalyst without pinhole defects (exposing the semiconductor) or aggregates (which diminish device performance).

In one embodiment, the electrocatalyst is deposited from an electrolyte by photodeposition, electrochemical deposition, or combination thereof.

In one embodiment, the electrode is formed by electrodepositing a conformal layer of an electrocatalyst from an electrolyte solution onto a surface of a photoanode.

In one embodiment, the semiconductor is an n-type semiconductor.

In one embodiment, the formed photoanode, when incorporated into a photoelectrochemical cell for electrolysis of water into oxygen, reduces a water electrolysis onset voltage compared to a second photoanode comprising the semiconductor without the electrocatalyst.

In one embodiment, the water electrolysis onset voltage is reduced by 50 mV to 400 mV.

In one embodiment, the first wavelength of the electromagnetic radiation is from 300 nm to 800 nm.

In one embodiment, the first irradiance of the electromagnetic radiation is from 0.1 W/m² to 1100 W/m², or the equivalent in pulsed irradiation.

In one embodiment, the electromagnetic radiation is selected from the group consisting of continuous radiation and pulsed radiation.

In one embodiment, the first electric bias is applied to the semiconductor as part of an electrochemical deposition system comprising a power source in electrical communication with the semiconductor and a counter electrode.

In one embodiment, the electrolyte solution comprises cations selected from the group consisting of cobalt, iridium, manganese, nickel, and ruthenium.

In one embodiment, the electrolyte solution comprises anions selected from the group consisting of phosphate, methyl phosphonate, borate, acetate, sulfate, and hydroxide.

In one embodiment, the semiconductor comprises a sensitizer having a sensitizer absorbance wavelength, said sensitizer absorbance being different from a semiconductor absorbance wavelength.

In one embodiment, the semiconductor comprises a group IV semiconductor having a formula selected from the group consisting of binary, ternary, and quaternary. In a further embodiment, the group IV semiconductor further comprises ions selected from the group consisting of cations and anions.

In one embodiment, the semiconductor comprises a material selected from the group consisting of an iron oxide, a zinc oxide, a titanium oxide, a tungsten-bismuth-vanadium oxide, a tungsten oxide, a gallium-zinc-oxide-nitride, or these materials also containing additional cations or anions.

In one embodiment, the combination of the first photoenergy and the first electric bias are sufficient to oxidize cations to deposit catalyst components from the electrolyte to form the solid conformal layer of the electrocatalyst.

In another embodiment, an electrode was made by deposition of cobalt catalyst onto mesostructured α-Fe₂O₃ from an electrolyte of Co²⁺. The deposition can be carried out by photodeposition or electrochemical deposition. Examples of the electrolyte include, without limitation, cobalt phosphate, cobalt borate, cobalt methyl phosphonate, cobalt nitrate, cobalt acetate, cobalt sulfate, and any combination thereof.

In one embodiment, the pH of the electrolyte is about 7 or higher. In one embodiment, the pH of the electrolyte was about 13 or higher. In another embodiment, the pH of the electrolyte was about 8.

In another embodiment, an electrode was made by electrochemical deposition of cobalt/phosphate catalyst (“Co-Pi”) onto mesostructured α-Fe₂O₃ and showed about 350 mV or higher cathodic shift of the onset potential for PEC water oxidation while retaining substantial photocurrent densities.

In another embodiment, Co-Pi was electrodeposited onto a mesostructured α-Fe₂O₃ photoanode. The photoelectrochemical properties of the resulting composite photoanodes were optimized for solar water oxidation under front-side illumination in pH 8 electrolytes. Relative to α-Fe₂O₃ photoanodes, more sparse deposition of Co-Pi onto the α-Fe₂O₃ resulted in a sustained five-fold enhancement in the photocurrent density and O₂ evolution rate at +1.0 V vs RHE.

In one embodiment, the photoanode comprises a photoanode material selected from the group consisting of an iron oxide, a zinc oxide, a titanium oxide, a bismuth vanadium oxide.

In one embodiment, the photoanode material has a physical shape selected from the group consisting of dendrites, wires, and belts. In another embodiment, said physical shape has nanoscopic dimensions. As used herein, the term nanoscopic dimensions refers to a shape having at least one feature (e.g., dendrites) having a smallest size of 100 nm or smaller.

In one preferred embodiment, the photoanode comprises hematite iron oxide dendrites. In a further preferred embodiment, the photoanode consists of hematite iron oxide dendrites conformally covered with a layer of cobalt phosphate.

In one embodiment, the electrocatalyst is selected from the group consisting of a cobalt-containing catalyst, an iridium-containing catalyst, a manganese-containing catalyst, a ruthenium-containing catalyst, a cobalt-containing oxygen evolving catalyst and a cobalt oxide catalyst.

In one preferred embodiment, the electrocatalyst is cobalt phosphate.

In one embodiment, the cathodic shift is from 50 mV to 400 mV.

In one embodiment, the layer of the electrocatalyst has a thickness of from 0.5 nm to 30 nm. Such a thickness is indicative of the “thin” nature of the conformal electrocatalyst coating. As set forth herein, such a thin coating is essential, not only to allow light through the electrocatalyst to the semiconductor, but also due to exacerbated electron-hole recombination with thicker catalyst films.

It will be appreciated that the range of 0.5 nm to 30 nm represents from about one molecular layer to about tens of molecular layers. Accordingly, it is preferred that a minimal number of molecular layers are used to conformally coat the semiconductor with electrocatalyst without pinhole defects (exposing the semiconductor) or aggregates (which diminish device performance).

In one embodiment, the electrocatalyst is deposited from an electrolyte by photodeposition, electrochemical deposition, or combination thereof.

In one embodiment, the electrode is formed by electrodepositing a conformal layer of an electrocatalyst from an electrolyte solution onto a surface of a photoanode.

In one embodiment, the formed photoanode, when incorporated into a photoelectrochemical cell for electrolysis of water into oxygen, reduces a water electrolysis onset voltage compared to a second photoanode comprising the semiconductor without the electrocatalyst.

In another aspect, an electrode is provided, comprising:

a photoanode; and

a competent electrocatalyst that causes a cathodic shift in the onset potential of the electrode.

In another aspect, an electrode is provided, comprising:

a α-Fe₂O₃ photoanode;

a competent electrocatalyst selected from the group consisting of cobalt catalyst, iridium catalyst, manganese catalyst, ruthenium catalyst, cobalt based oxygen evolving catalyst and cobalt oxide catalyst.

In another aspect, an electrode is provided, comprising:

a α-Fe₂O₃ photoanode;

a competent electrocatalyst comprising a cobalt catalyst deposited onto the α-Fe₂O₃ photoanode from an electrolyte of Co²⁺ by photodeposition, electrochemical deposition, or combination thereof,

wherein the electrolyte comprises a composition selected from the group consisting of cobalt phosphate, cobalt nitrate, cobalt acetate, cobalt sulfate, and any combination thereof; and the electrode having about a several hundred millivolt cathodic shift of the onset potential for PEC water oxidation.

A system/device for converting water to hydrogen using only sunlight as an energy source is provided. The system includes a PEC comprising a photoanode formed using photoelectrochemical deposition and a photovoltaic cell. As described elsewhere herein, a water-splitting PEC typically requires over 1 V to produce hydrogen and oxygen from water, which is an electrical requirement that cannot be met by present PV technology. However, using the photoelectrochemical deposition method provided herein, the cathodic shift achieved in improving present photoanodes for PEC (e.g., Co-Pi/hematite), makes efficient sub-1 V water splitting in a PEC possible. Accordingly, by combining a PV cell with a PEC system having a photoanode formed using a photoelectrochemically deposited electrocatalyst on a semiconductor results in a system for converting water to hydrogen using only sunlight.

The following examples are provided to better illustrate the claimed invention and are not to be interpreted in any way as limiting the scope of the invention. All specific compositions, materials, and methods described below, in whole or in part, fall within the scope of the invention. These specific compositions, materials, and methods are not intended to limit the invention, but merely to illustrate specific embodiments falling within the scope of the invention. One skilled in the art may develop equivalent compositions, materials, and methods without the exercise of inventive capacity and without departing from the scope of the invention. It will be understood that many variations can be made in the procedures herein described while still remaining within the bounds of the invention. It is the intention of the inventors that such variations are included within the scope of the invention.

EXAMPLES Example 1 α-Fe₂O₃ Photoanode Fabrication

Mesostructured Si-doped α-Fe₂O₃ photoanodes of 400-500 nm thickness were grown by atmospheric pressure chemical vapor deposition (APCVD) using Fe(CO)₅ and tetraethoxysilane (TEOS) as precursors, delivered to an FTO substrate at 470° C. using Ar carrier gas. SEM images of a representative α-Fe₂O₃ photoanode are shown in FIGS. 1A and 1B and revealed a highly structured electrode surface. PEC measurements were then performed in a 3-electrode configuration using an aqueous OH⁻ electrolyte (1 M NaOH, pH 13.6), a Pt counter electrode, and Ag/AgCl as the reference electrode. Photocurrent densities were measured as a function of applied voltage under simulated 1 sun AM1.5 solar irradiation. The α-Fe₂O₃ PEC performance was found to depend strongly on surface morphology, Si doping level, and growth temperature, among other parameters.

Example 2 Fabrication of Co-Pi/α-Fe₂O₃ Photoanodes

Mesostructured Si-doped α-Fe₂O₃ photoanodes were grown on F:SnO₂ (FTO)-coated glass substrates (TEC15, 15 Ω/cm² Hartford Glass Co.) by atmospheric pressure chemical vapor deposition (APCVD) following procedures known in the art. The precursors, Fe(CO)₅ (Aldrich 99.999%) and TEOS (Aldrich 99.999%), were delivered by bubbling Ar gas (Praxair, 5.0 Ultra High Purity) at 11.3 and 19.4 mL/min, respectively, controlled by mass flow controllers. The gas was then mixed with air flowing at 2 L/min and directed by a glass tube onto the lower portion of a 50×13×2.3 mm³ FTO substrate kept at 470° C. The Co-Pi catalyst was electrodeposited onto the oxide anodes as known in the art. The anode was submerged in a buffer solution of 0.1 M potassium phosphate (pH 7) containing 0.5 mM Co(NO₃)₂ and a bias of 1.29 V (vs. NHE) was applied for 1 hr. For PEC measurements, the anode surface was masked during electrochemical deposition to yield a catalyst-covered area that matched the irradiated area (Ø=6 mm). Masking was achieved using electrical tape, which was then removed for PEC measurements. Composite Co-Pi/α-Fe₂O₃ anodes for which the mask was not used showed greater dark currents from the Co-Pi catalyst, but were otherwise very similar.

Electronic absorption spectra were measured using a Cary 500 UV/vis/NIR spectrophotometer (Varian). SEM images were collected using a FEI Sirion scanning electron microscope operating at 5 kV. Electrochemical measurements were performed in a 3-electrode configuration using an aqueous hydroxide electrolyte (1 M NaOH, pH 13.6), a Pt counter electrode, and an Ag/AgCl reference electrode. In a typical measurement, a titanium clasp was used to make contact with the upper 25% of the 5 cm long anode, where no α-Fe₂O₃ had been deposited. The bottom ˜50% of the anode was submerged in the electrolyte solution in a home-built optical cell. Cyclic voltammetry measurements were performed using a computer-controlled Eco Chemie μAutolab II potentiostat. Potentials are reported vs both Ag/AgCl and RHE, the latter obtained using the formula E_(RHE)=E_(AgCl)+0.059 pH+0.1976V. Photocurrent densities were measured as a function of applied voltage under simulated AM1.5 solar irradiation (1 sun), achieved using an Oriel 96000 solar simulator integrating a 150 W Xe arc lamp and Oriel 81094 filter, and delivered to the anode via fiber optic. Measurements were performed at a scan rate of 50 mV/s. IPCE measurements were performed using a Xe arc lamp with an Oriel Cornerstone 74000 monochromator with slits set to ˜10 nm spectral bandwidth at the designated bias voltage provided by the potentiostat. The wavelength was scanned at 1 nm/s. Photon power densities were determined using a calibrated Si photodiode. Dark current measurements probe the entire submerged FTO+α-Fe₂O₃ (or Co-Pi/α-Fe₂O₃) surface, whereas photocurrents represent the response achieved from just the irradiated area normalized to 1 cm². This area was circular with a diameter of 6 mm. Typical monochromatic photon power densities in the IPCE measurements were ˜0.50 W/m². For the data shown in FIGS. 2A-2C, the α-Fe₂O₃ photoanode data were collected first, then the Co-Pi catalyst was deposited onto the same α-Fe₂O₃ photoanode, and then the parallel data were collected on the Co-Pi/α-Fe₂O₃ photoanode.

FIG. 2A shows dark (dashed) and photocurrent (solid) densities for an α-Fe₂O₃ photoanode with back-side illumination. Whereas the dark response was negligible up to 1.5 V vs RHE, the photoresponse showed a rise and plateau with an onset voltage of ˜1 V vs RHE that typifies α-Fe₂O₃. FIGS. 1C and 1D showed SEM images of a representative α-Fe₂O₃ photoanode following Co-Pi electrochemical deposition for 1 hr as known in the art. Extensive cracking of the ˜200 nm thick catalyst layer occurred upon drying for the SEM measurement. FIG. 1D showed a portion of the catalyst layer that curled off of the α-Fe₂O₃ film upon drying, revealing its underside. This image showed the inverse mesostructure from the α-Fe₂O₃ anode, demonstrating that the catalyst layer conformed to the topology of the α-Fe₂O₃ surface. A high degree of interfacial contact between the α-Fe₂O₃ and catalyst layers was achieved. FIG. 2A also showed the dark and photocurrent responses of the Co-Pi α-Fe₂O₃ composite photoanode prepared by electrochemical deposition of the Co-Pi catalyst on the same α-Fe₂O₃ photoanode. The major phenomenological observation was that modification of α-Fe₂O₃ with Co-Pi reduced the bias voltage required for solar PEC water oxidation by about 350 mV or higher, corresponding to a reduction from ˜1.2 to about ˜0.9 V or lower that would be required from the PV of a water splitting PEC/PV tandem cell.

At 1.4 V (RHE), α-Fe₂O₃ photocurrent densities with front-side illumination were approximately 2× greater than with back-side illumination (FIG. 3), without being bound by any theory, a common observation attributable to the greater surface area of the anode front. In the Co-Pi/α-Fe₂O₃ anodes, however, front-side illumination did not greatly enhance the photocurrent, without being bound by any theory, likely because of non-productive absorption by the catalyst layer. Co-Pi absorbed throughout the visible spectral region (FIG. 2B) but generated no detectable photocurrent, either on α-Fe₂O₃ or directly on FTO. IPCE measurements of the α-Fe₂O₃ (1.23 V vs RHE) and Co-Pi/α-Fe₂O₃ (1 V vs RHE) photoanodes using back-side illumination showed essentially identical dispersion (FIG. 2C), without being bound by any theory, in both cases deriving only from α-Fe₂O₃ excitation. Co-Pi thus behaved solely as a surface electrocatalyst. The composite photoanode of FIG. 2C showed IPCE about 15% or higher at 550 nm and 1 V vs RHE, conditions where α-Fe₂O₃ alone showed negligible photocurrent (FIG. 2 a). This IPCE maximized at 450 nm (18%) before decreasing again below ˜400 nm because of the decreasing light penetration depth (FIG. 2B).

To test for the possibility that the cathodic photocurrent shift came from the action of solvated cobalt as a redox mediator, a set of control experiments involving deliberate addition of solvated Co²⁺ was performed by adding Co(OH)₄ ²⁻ to the 1 M NaOH electrolyte of the PEC cell. Co(OH)₄ ²⁻ was prepared by dissolving cobalt nitrate in a 50 wt % concentrated NaOH aqueous solution to make a ˜0.005M Co(OH)₄ ²⁻ solution, which was then added to distilled water to reach pH ˜13. The final solution was added dropwise to the electrolyte of the PEC cell under operating conditions, where its influence on dark and photocurrent densities of various photoanodes could be monitored. Although sparingly soluble at pH 13.6, the precipitation of solid Co(OH)₂ from the electrolyte solution was likely slow, as indicated by observation of the characteristic Co(OH)₄ ²⁻⁴T₁(P) ligand field band in the absorption spectrum of the pH ˜13 stock solution even ˜30 min after preparation (FIG. 4), which is roughly three times longer than required to collect the PEC data. Although solvated Co(OH)₄ ²⁻ was difficult to detect spectroscopically in the PEC cell at very low concentrations, its presence was readily detected electrochemically by an increase in dark current for both FTO and α-Fe₂O₃-modified FTO anodes. When Co(OH)₄ ²⁻ was added to the PEC cell during α-Fe₂O₃ film measurement, an increase in dark current was observed but no significant change in photocurrent resulted, arguing against interpretation of the cathodic shift described in the manuscript as arising from participation of a soluble Co²⁺ redox mediator. Independent control experiments in which I-V scans of unmodified FTO anodes were measured before and after repeated photocurrent measurements on Co-Pi/α-Fe₂O₃ composite photoanodes without replacing the electrolyte also failed to detect solvated Co²⁺, again arguing against interpretation of the cathodic photocurrent shift described in the manuscript as arising from participation of a soluble Co²⁺ redox mediator.

As the control experiments showed, the possibility of dissolved cobalt acting as redox mediator, or of an unidentified sacrificial reagent contributing to photocurrent, was eliminated by the following observations: (i) Addition of solvated Co²⁺ to the electrolyte had no noticeable effect on photocurrent densities; (ii) Replacement of the PEC electrolyte solution with new stock solution caused no change in photocurrent and did not lead to a photocurrent induction period; (iii) Continuous photocatalysis at 1 V vs RHE for about 10 hrs or longer showed no change in performance. Therefore, the cathodic shift in FIG. 2A reflected the ability to drive Equation 1 (of the Background section) at much smaller overpotentials using the composite photoanodes than with α-Fe₂O₃ alone.

Most α-Fe₂O₃ PEC cells operating under similar conditions show negligible photocurrent densities below 1 V vs RHE. Modification of the α-Fe₂O₃ surface by adsorption of Co²⁺ from aqueous 10 mM Co(NO₃)₂ was previously shown to cause an ˜17% increase in current density at 1.23 V vs RHE and an 80 mV cathodic shift of the onset potential. Similarly, growth of RuO₂ onto α-Fe₂O₃ surfaces led to a 120 mV cathodic shift of the onset potential with about 80 μA/cm² or lower at 1 V vs RHE. Interestingly, α-Fe₂O₃ nanorods have shown greater relative photocurrent densities at low bias than typical mesostructured α-Fe₂O₃ photoanodes, but with photocurrent densities of ˜2 μA/cm². Without being bound to any theory, it is possible that the conformal catalyst deposition facilitates interfacial hole transfer from α-Fe₂O₃ to Co-Pi, allowing photon absorption and redox catalysis to be effectively decoupled while retaining photocurrent densities. Efficient hole transfer from α-Fe₂O₃ to Co-Pi should enhance the electron gradient in the α-Fe₂O₃ mesostructure under irradiation, also contributing to the driving force for electron diffusion to the FTO and reducing deleterious carrier recombination processes. Catalyst electrochemical deposition onto α-Fe₂O₃ may also passivate surface defects.

The experimental results for the Co-Pi/α-Fe₂O₃ composite photoanodes may be summarized in FIG. 5 (and this model can be used to describe the behavior of all composite photoanodes provided in the disclosed aspects and embodiments herein). Referring to FIG. 5, photoexcitation of α-Fe₂O₃ generates an electron-hole pair. Photogenerated holes are trapped by the Co-Pi catalyst, which excels at water oxidation. Photogenerated electrons migrate to the FTO back contact and pass through the circuit to the Pt counter electrode, where water reduction occurs in the 3-electrode configuration. The present results demonstrate that partitioning photoabsorption, charge separation, and redox catalysis in composite photoanodes offers promising opportunities for improving solar water-splitting PECs.

Si doped α-Fe₂O₃ photoanodes were fabricated on fluorine doped tin oxide (FTO) glass (50×13×2.3 mm TEC 15 Hartford Glass Co.) at 470° C. for 5 min by atmospheric pressure chemical vapor deposition (APCVD) following procedures known in the art. The α-Fe₂O₃ films investigated here were typically ˜400-500 nm thick.

For Co-Pi deposition onto α-Fe₂O₃ photoanodes for the following data, electrical tape with an aperture that matched the irradiated area during photoelectrochemical (PEC) experiments (Ø=6 mm diameter) was applied onto the α-Fe₂O₃. As the working electrode, α-Fe₂O₃ was submerged into a solution of 0.5 mM cobalt nitrate in 0.1 M pH 7 potassium phosphate (KPi) buffer. A Pt mesh was used as the counter electrode and saturated Ag/AgCl was used as the reference electrode. Co-Pi was electrodeposited at +1.1 V vs Ag/AgCl for 15 (FIG. 6) or 30 (FIG. 7) min. Typical current densities during deposition were ˜20-200 μA/cm² (FIGS. 6 and 7) For electrolysis studies involving Co-Pi on FTO, Co-Pi was electrodeposited for 15 min using the above conditions and a mask of 1 cm×1 cm.

Photoelectrochemical experiments. Current-voltage characteristics were measured using an Eco Chemie μ-Autolab II potentiostat in a home-built three-electrode optical cell using Ag/AgCl as the reference electrode and a Pt wire as the counter electrode. Contact to the photoanodes was made by a titanium clasp attached to the exposed FTO surface at the top of the anode, while the lower portion containing the sample was submerged in the electrolyte. Measurements were performed in 1 M NaOH(aq) at pH 13.6, 0.1 M KPi buffered at pH 8, and 0.1 M NaCl(aq) buffered at pH 8 with 0.1 M KPi. Potentials are reported vs Ag/AgCl (measured) or RHE (obtained using the relationship E_(RHE)=E_(Ag/AgCl)+0.0591*pH+0.1976 V). Photocurrent densities were measured under 1 sun, AM 1.5 simulated sunlight using an Oriel 96000 solar simulator equipped with a 150 W Xe arc lamp and an Oriel 81094 filter. The photoanodes were masked to illuminate a circular area of 6 mm diameter. Power dependence measurements were performed using an Ag variable neutral density filter, Thorlabs NDC-50C-2M. Unless otherwise stated, all films in this example were illuminated from the front side of the photoanode. Unless otherwise specified, all experiments in this example were performed at room temperature in air atmosphere.

Oxygen detection. The detection of O₂ was performed using a YSI 5000 dissolved oxygen meter equipped with a YSI 5010 self-stirring Clark-type probe in a three-neck flask with an optical window. Before use, the electrolyte (0.1 M KPi buffered at pH 8) was degassed and purged with argon gas. Measurements were conducted in argon in the same three-electrode configuration described for PEC experiments using the same light source. Again, the photoanodes were masked to illuminate a circular area of 6 mm in diameter. Consecutive measurements were taken at +1.0, 1.1, and 1.23 V vs RHE for two hours at each potential. While the light was off between voltages (˜160 seconds), there was no increase and sometimes even a decrease in the O₂ level due to consumption by the Clark electrode.

Co-Pi/α-Fe₂O₃ photoanode performed under front-side illumination and mild pH conditions.

Optimization of the composite photoanodes for front-side illumination was carried out at mild pH conditions. To reduce photon absorption by the catalyst, Co-Pi deposition times were decreased from the original one-hour duration.

The basic electrolyte (pH 13.6) used previously herein is generally undesirable for practical applications. A gradual decrease in photocurrent density from α-Fe₂O₃ anodes alone was observed under continuous illumination in 0.1 M KPi electrolyte at pH 7 and +1.3 V vs RHE (FIG. 8). Therefore PEC measurements were carried out in electrolytes at pH 8, which is around the pH of natural seawater. Two approaches were used to achieve this pH. One involved use of 0.1 M potassium phosphate (KPi), buffered to pH 8. The second involved 0.1 M NaCl buffered to pH 8 with 0.1 M KPi.

FIGS. 9A and 9B show current-voltage (J-V) curves collected for Co-Pi/α-Fe₂O₃ composite photoanodes prepared with 30 min deposition of Co-Pi and measured in various electrolytes. Each data set was compared to analogous data collected for the same Fe₂O₃ film measured before Co-Pi deposition. FIG. 9A shows the J-V curves collected using 1 M NaOH at pH 13.6 and FIG. 9B shows data collected using 0.1 M KPi at pH 8. As described above, in 1 M NaOH, Co-Pi deposition yields a cathodic shift of about 350 mV or higher in the photocurrent onset potential relative to α-Fe₂O₃ (FIG. 9A). PEC measurements in 0.1 M KPi electrolyte at pH 8 also showed similar shifts. With front-side illumination there was a slight decrease in the photocurrent density at high applied potentials (+1.3-1.6 V) compared to α-Fe₂O₃ alone, attributed to partial photon absorption by the catalyst layer. The data in FIGS. 9A and 9B demonstrated that Co-Pi/α-Fe₂O₃ composite photoanodes can operate under reduced pH conditions and with front-side illumination.

Kinetic bottleneck in Co-Pi/α-Fe₂O₃ composite photoanodes. In the course of efforts to optimize the Co-Pi/α-Fe₂O₃ composite photoanodes, it was recognized that improvements in efficiency were often accompanied by increasingly apparent symptoms of kinetic limitations. For example, FIGS. 9A and 9B also showed J-V curves of the same Co-Pi/α-Fe₂O₃ composite photoanodes measured at the slower scan rate of 10 mV/s (dashed line). The open circles in FIGS. 9A and 9B were the quasi-steady state photocurrent densities measured after 200 s of simulated solar irradiation at each applied potential. The cathodic shift and photocurrent densities both decreased as the scan rate was slowed, converging on those of the underlying α-Fe₂O₃ at slowest scan rates. Upon increasing the scan rate again, the J-V curves recovered their original shape, even after 10+ hours of continuous illumination. Illuminating with chopped light also recovers the photocurrent enhancement. This behavior is largely independent of electrolyte or pH (FIGS. 9A and 9B), and suggests the existence of a kinetic bottleneck in the performance of these Co-Pi/α-Fe₂O₃ composite photoanodes.

To detail this kinetic bottleneck, its symptoms were explored in various complementary measurements on a single Co-Pi/α-Fe₂O₃ composite photoanode in 0.1 M KPi electrolyte at pH 8. The resulting data are summarized in FIGS. 10A and 10B. Expanding on FIGS. 9A and 9B, photocurrents were measured at a greater variety of scan rates to show the evolving characteristics of the J-V curves. With faster scan rates, the first maximum shifted to higher potentials. The inset showed the photocurrent response vs time upon unblocking the light path, measured at +1.1 V vs RHE. A large initial spike in photocurrent upon illumination was followed by multi-exponential decay to a lower steady-state current density with an effective time constant on the order of 10 sec, i.e., comparable to the data collection timescale (10s of seconds). Without being bound to any theory, it is believed that some of the initial current density in this trace is attributed to cobalt oxidation, which is an essential step in the water oxidation mechanism. Oxygen detection experiments in 0.1 M KPi at 1 V vs RHE showed that this initial high current density was accompanied by a spike in the oxygen evolution rate, however, which then also decreased as the current density decays (FIG. 11). The spike and subsequent current density decay therefore cannot be ascribed solely to an initial charging current for the redox active Co-Pi layer. FIG. 10B plots the steady-state photocurrent density for a Co-Pi/α-Fe₂O₃ composite photoanode measured as a function of illumination power density between 0 and 1 sun. There was a marked saturation in the photocurrent as the light intensity was increased.

Overall, four major symptoms of this kinetic bottleneck can be identified: (i) a scan rate dependence, (ii) a kinetic decay in the photocurrent density, (iii) photocurrent saturation upon increased illumination, and (iv) a sweep-rate-dependent maximum at the beginning of the J-V curve. Without being bound by any theory, the sweep-rate dependence of this maximum is a consequence of the superposition of an increasing current density from increasing bias with a current decay.

Parallel measurements were performed for α-Fe₂O₃ photoanodes alone under the same experimental conditions. Under the conditions represented in FIGS. 9A and 9B, the J-V curves of α-Fe₂O₃ photoanodes did not change significantly with scan rate (data collected at 10 mV/s are plotted in FIGS. 9A and 9B). In time-dependence measurements, irradiation typically induced a small initial current spike followed by relaxation to a similar steady-state value within a few seconds. Finally, the α-Fe₂O₃ photocurrents increased much more linearly with increasing light intensity under these conditions (FIG. 10B). Without being bound by any theory, it is believed that the kinetic bottleneck is associated with the Co-Pi modification, perhaps as an intrinsic limitation of the catalyst under these conditions, or perhaps because of slow interfacial electron transfer.

To test the catalyst alone, Co-Pi was electrodeposited on FTO and electrochemical experiments were conducted in 0.1 M KPi electrolyte buffered to pH 7 with stirring. FIG. 12 showed the J-V characteristics of Co-Pi at various scan rates, and the current density time dependence under typical electrolysis conditions of +1.1 V vs Ag/AgCl. Like in FIGS. 10A and 10B, the bulk electrolysis by Co-Pi on FTO also showed a scan rate dependence and a decay in the current density in the region where water oxidation was normally observed, +1.3 V vs NHE, or +1.7 V vs RHE. These observations support that the kinetic bottleneck observed in the Co-Pi/α-Fe₂O₃ photoanodes was associated with the Co-Pi catalyst itself rather than with the Co-Pi/α-Fe₂O₃ interface.

Alleviating the kinetic problem in Co-Pi/α-Fe₂O₃. Without being bound to any theory, if there is a kinetic bottleneck in Co-Pi/α-Fe₂O₃ photoanodes, it is possible that even further reduction of the Co-Pi deposition time may remediate the problem. For example, thick layers of Co-Pi may inhibit rapid charge or proton transport from electrolyte through the catalyst, thus restricting current flow and allowing other non-productive recombination pathways to become competitive. To test this possibility, Co-Pi was electrodeposited on α-Fe₂O₃ photoanodes for 15 min. FIGS. 13A-13D showed SEM images of the resulting Co-Pi/α-Fe₂O₃ composite photoanode. Unlike the dense coverage of Co-Pi on α-Fe₂O₃ after 1 hr of electrochemical deposition described above (FIG. 1C), 15 min deposition resulted in sparse coverage of the α-Fe₂O₃ photoanode by Co-Pi, which displayed ring-like patterns on the surface (FIG. 13A). These ring patterns were formed from smaller patches of Co-Pi (FIGS. 13B-13D). These patches were estimated to be thinner than 100 nm, compared to ˜200 nm thick film that was deposited during 1 hour deposition (FIG. 1C), and they showed cracking due to drying. The Co-Pi layer conformed to the topology of the α-Fe₂O₃ well and the microstructure of the α-Fe₂O₃ surface could be seen through the catalyst in places. Without being bound by any theory, it is hypothesized that these patches were somehow associated with scratches or pinholes in the α-Fe₂O₃ that allowed current to flow more readily during electrochemical deposition. Preliminary energy dispersive X-ray analysis (EDAX) experiments demonstrated the existence of a very thin catalyst layer over the entire α-Fe₂O₃ surface, and without being bound by any theory, it is possible that catalysis was also distributed over the entire surface.

PEC measurements were performed on these thinly covered Co-Pi/α-Fe₂O₃ photoanodes and the results were shown in FIGS. 14A and 14B. Cathodic shifts of ˜180 mV were observed, as were enhanced photocurrents across the entire potential range. The J-V curves measured in NaOH electrolytes no longer exhibited the marked scan rate dependence that was observed for the parallel set of photoanodes with greater Co-Pi coverage (FIGS. 9A and 9B). With a thinner layer of Co-Pi, the enhanced current density is maintained even after 200 sec (FIGS. 14A and 14B), and the cathodic shift is stable. For PEC measurements conducted in 0.1 M KPi at pH 8, a gradual decay in the photocurrent over time was still evident, and the cathodic shift decreased from ˜200 mV to 150 mV after 200 sec of continuous illumination (FIG. 14B). PEC measurements were also performed in 0.1 M NaCl buffered to pH 8 with KPi. The resulting J-V curves with NaCl added are essentially indistinguishable from those without NaCl (FIG. 14B), demonstrating that the present of chloride did not interfere with PEC water oxidation with Co-Pi/α-Fe₂O₃ composite photoanodes.

FIGS. 15A and 15B compared the kinetic responses of Co-Pi/α-Fe₂O₃ photoanodes with thicker (FIG. 15A, 30 min deposition, see FIGS. 16A-16D) and thinner (FIG. 15B, 15 min deposition, see FIGS. 13A-13D) Co-Pi coverage. The photocurrent decay curves of FIG. 15A showed a large initial spike in current density, followed by a multiexponential decrease with T ˜10 sec to a small steady-state photocurrent density close to that of the underlying α-Fe₂O₃ photoanode. In contrast, the photoanodes with thinner Co-Pi coverage showed substantially more stable performance. The steady-state photocurrent densities in FIG. 15B were enhanced relative to those of the parent α-Fe₂O₃ photoanodes. The results showed a sustainable photocurrent density that was enhanced relative to α-Fe₂O₃ by more than an order of magnitude at 0.83 V, where α-Fe₂O₃ alone did not exhibit significant photocurrent (FIG. 15B). Gains in photocurrent were less substantial at higher applied potentials, without being bound by any theory, it was likely due to contributions directly from α-Fe₂O₃. FIGS. 14A, 14B, 15A, and 15B demonstrated that reduced Co-Pi deposition onto α-Fe₂O₃ photoanodes circumvented the major kinetic limitations identified above, while still shifting the onset potential of α-Fe₂O₃ by ˜180 mV, and simultaneously facilitated front-side illumination for maximum photocurrent densities.

Decreased deposition of Co-Pi onto α-Fe₂O₃ largely overcame the kinetic limitations described in FIGS. 10A and 10B, but there was still some evidence of such kinetic effects in KPi electrolyte (FIGS. 14B, 15B) that were not observed in 1 M NaOH. For 1 M NaOH, there was a small initial spike in the photocurrent followed by a small gradual increase to steady state. Without being bound by any theory, it is possible that limited mobility of protons through the amorphous catalyst may contribute to the kinetic bottleneck described by FIGS. 9A, 9B, 10A, 10B, 12A, and 12B, and that OH⁻ is better able to overcome this limitation. Overall, FIGS. 14A, 14B, 15A, and 15B showed that this bottleneck was lessened by changing the electrolyte from pH 8 KPi to pH 13.6 NaOH, and was effectively circumvented by reducing the density of catalyst on the α-Fe₂O₃ surface.

Oxygen evolution. In addition to current density measurements, PEC O₂ evolution by the Co-Pi/α-Fe₂O₃ composite photoanodes was also examined. Oxygen evolution was measured at various applied potentials before and after 15 min of Co-Pi electrochemical deposition onto an α-Fe₂O₃ photoanode. Measurements were performed in 0.1 M KPi electrolyte at pH 8. FIG. 17A showed the J-V characteristics of the α-Fe₂O₃ photoanode used for these measurements, before and after Co-Pi deposition, and for both front and back-side illumination. Photocurrent densities increased substantially with front-side illumination, particularly at low potentials. FIG. 17B plotted the photocurrent density vs time along with the O₂ concentrations measured simultaneously using the Clark-type electrode.

Sustained photocurrent was observed for the Co-Pi/α-Fe₂O₃ composite photoanode over the course of this ˜6 hour experiment. This steady-state photocurrent was enhanced over that of the parent α-Fe₂O₃ film, even after several hours of illumination, and was accompanied by a correspondingly large enhancement in the O₂ evolution rate. The photocurrent density and O₂ evolution enhancement factors

$\left( {{\frac{J\left( {{{{Co}—{Pi}}/\alpha} - {{Fe}_{2}O_{3}}} \right)}{J\left( {\alpha - {{Fe}_{2}O_{3}}} \right)}\mspace{14mu} {and}\mspace{14mu} \frac{{\left\lbrack O_{2} \right\rbrack}/{{t\left( {{{{Co}—{Pi}}/\alpha} - {{Fe}_{2}O_{3}}} \right)}}}{{\left\lbrack O_{2} \right\rbrack}/{{t\left( {\alpha - {{Fe}_{2}O_{3}}} \right)}}}},} \right.$

respectively) measured at each applied potential were indicated in FIG. 17B. The amount of dissolved O₂ detected by the Clark-type electrode was lower than the theoretical maximum for the measured current densities, but without being bound by any theory, this difference may be attributable to the adherence of bubbles on the rough surface of the Co-Pi/α-Fe₂O₃ photoanode. Occasional jumps in the photocurrent density were observed for the composite photoanodes and may be related to release of these bubbles.

FIG. 17B showed that PEC O₂ evolution by the Co-Pi/α-Fe₂O₃ composite photoanode was enhanced over that of the same α-Fe₂O₃ photoanode without Co-Pi. Despite the gradual decline in photocurrent density for these photoanodes when measured in 0.1 M KPi electrolyte at pH 8 (FIGS. 14A, 14B, 15A, and 15B), after 2 hours of continuous irradiation at +1.0 V vs RHE, ˜5 times more oxygen was produced for the Co-Pi modified α-Fe₂O₃ photoanode with no detectable degradation in performance.

Example 3 Co-Pi/α-Fe₂O₃ Photoanodes Prepared by Photoelectrochemical Deposition

Co-Pi catalyst was photoelectrochemical deposited onto α-Fe₂O₃ photoanodes by using light and an external applied bias to deposit Co-Pi. Without being bound by theory, in principle, photogenerated holes can be used to oxidize Co²⁺ from the electrolyte to form Co-Pi on the α-Fe₂O₃ photoanode and the electron can be removed by water reduction. Because photogenerated electrons in the conduction band of α-Fe₂O₃ are below the energy needed to reduce protons to hydrogen, a very low bias was applied to assist in photoelectrochemical deposition of Co-Pi. Any light source with sufficient energy to excite the band gap of α-Fe₂O₃ can be used in a photoelectrochemical deposition on α-Fe₂O₃. In this embodiment, photoelectrochemical deposition on α-Fe₂O₃ was conducted in a three-electrode configuration from a solution of Co²⁺ in potassium phosphate (KPi) buffer under 1 sun AM 1.5 simulated solar irradiation. A Pt mesh was used as the counter electrode and saturated Ag/AgCl was used as the reference electrode. Typical current densities during deposition were ˜1-100 μA/cm².

FIG. 18 shows a ˜120 mV cathodic shift in the J-V curve of an α-Fe₂O₃ photoanode after Co-Pi deposition measured in 0.1 M KPi at pH 8. Photoelectrochemical deposition and electrochemical deposition, of Co-Pi had similar effect of shifting the onset potential for water oxidation of α-Fe₂O₃.

In another exemplary embodiment, a photo-assisted electrochemical deposition approach (i.e., photoelectrodeposition) was used to deposit a cobalt-phosphate water oxidation catalyst (“Co-Pi”) onto dendritic mesostructures of α-Fe₂O₃. A comparison between this approach, electrochemical deposition of Co-Pi, and Co²⁺ wet impregnation showed that photo-assisted electrochemical deposition of Co-Pi yields superior α-Fe₂O₃ photoanodes for photoelectrochemical water oxidation. Stable photocurrent densities of 1.0 mA/cm2 at 1.0 V and 2.8 mA/cm² at 1.23 V vs RHE measured under standard illumination and basic conditions were achieved. By allowing deposition only where visible light generates oxidizing equivalents, photo-assisted electrochemical deposition provides a more uniform distribution of Co-Pi onto α-Fe₂O₃ than obtained by electrochemical deposition. This approach of fabricating catalyst-modified metal-oxide photoelectrodes may be attractive for optimization in conjunction with tandem or hybrid photoelectrochemical cells.

By way of background, the maturation of photoelectrochemical (PEC) water splitting as a viable solar fuels technology has been hindered by the need to identify photoelectrode materials that are simultaneously efficient at solar energy conversion, stable under reaction conditions, and inexpensive. Whereas high solar-to-hydrogen conversion efficiencies of 12.4% have been demonstrated using semiconductor multilayer devices, these efficiencies are not sustainable even on the one-day timescale because of rapid electrode decomposition. Metal oxides have been widely studied as chemically robust alternatives, beginning with TiO₂, but have been limited by various factors including low carrier mobilities, low absorption coefficients, or poor catalytic proficiencies. Hematite (α-Fe₂O₃) has emerged as a prototype photoanode for PEC water oxidation because of its balance of visible light absorption (bandgap of 2.1 eV), chemical stability, low cost, and large positive valence band edge potential. Low mobilities (10⁻²-10⁻¹ cm² V⁻¹ s⁻¹) and short hole diffusion lengths (2-4 nm or 20 nm) have generally led to low PEC water oxidation efficiencies in bulk α-Fe₂O₃, but doping and nanostructuring have been used to sidestep these shortcomings, by increasing carrier density, decreasing the distance minority carriers have to travel to reach the reactive surface, and increasing semiconductor-electrolyte interfaces. Doping with silicon has been suggested to increase photocurrent densities by several orders of magnitude in mesostructure α-Fe₂O₃ films. Nanowires and nanotubes of α-Fe₂O₃ have also shown increased photocurrent densities relative to bulk, although such structures have so far been limited to absolute one-sun current densities on the order of μA/cm².

Interfacing such mesostructured metal-oxide photoanodes with competent water oxidation catalysts offers one approach to improving their performance. Similar to Nature's photosynthesis, the separation of photon absorption, charge separation, and water oxidation tasks in composite photoelectrodes allows components performing each task to be optimized independently and thereby enables a greater flexibility in the selection of component materials.

Electrochemical deposition of Co-Pi, as described herein (e.g., Example 2), forms an adequate junction between the catalyst and semiconductor for interfacial charge transfer, and the resulting Co-Pi/α-Fe₂O₃ composite photoanodes are stable under photolysis conditions. A kinetic bottleneck was observed with thick layers of Co-Pi that hindered the steady-state turnover of the composite photoanodes, especially at low applied potentials. This kinetic limitation was remediated by reducing the Co-Pi coverage, but at the expense of overpotential. With short electrochemical deposition times, however, Co-Pi was found to deposit preferentially at pinholes, scratches, or other imperfections in the α-Fe₂O₃ film, where more current can flow from the underlying conductive FTO substrate. This inhomogeneity affects the performance of Co-Pi/α-Fe₂O₃ photoanodes by creating areas where the catalyst layer is too thick (kinetic bottleneck), and it influences the reproducibility of the Co-Pi deposition itself. Ultimately, a stable and efficient water oxidation photoanode is desired, and methods to apply a uniform thin catalyst layer onto highly mesostructured metal-oxide photoanodes, such as α-Fe₂O₃ are therefore needed.

In this Example, we describe photo-assisted electrochemical deposition (“photoelectrodeposition”) of Co-Pi onto mesostructured α-Fe₂O₃ photoanodes, and present a comparison between this approach, electrochemical deposition of Co-Pi and Co²⁺ adsorption. These three approaches are summarized in FIG. 21A-FIG. 21C. Among these, photo-assisted electrochemical deposition of Co-Pi is found to yield superior PEC performance by all metrics, including absolute onset potential, cathodic shift of the onset potential, and maximum current density. In combination with recently improved dendritic α-Fe₂O₃ photoanodes, the photo-assisted electrochemical deposition of Co-Pi yields arguably the best overall α-Fe₂O₃ photoanodes for PEC solar water splitting reported to date, with stable current densities of 1.0 mA/cm² at 1.0 V and 2.8 mA/cm² at 1.23 V vs RHE measured under standard 1 sun, AM 1.5 illumination conditions at pH 13.6.

Mesostructured α-Fe₂O₃ photoanodes were fabricated on FTO glass by the APCVD method described in Example 2. Masks with apertures of 6 mm in diameter were applied to define the active surface areas. Co-Pi was electrodeposited onto α-Fe₂O₃ photoanodes by modification of published procedures. A three-electrode cell was used with α-Fe₂O₃ as the working electrode, Ag/AgCl as the reference electrode, and Pt mesh as the counter electrode. 0.9 V vs Ag/AgCl was applied in a solution of 0.5 mM cobalt nitrate in 0.1 M potassium phosphate buffer at pH 7. The amount of Co-Pi deposited was controlled by the deposition time, which ranged between 200-500 s. Current densities were typically ˜2-10 μA/cm² during deposition.

Photo-assisted electrochemical deposition of Co-Pi onto mesostructured α-Fe₂O₃ was performed from the same electrolyte composition used for electrochemical deposition, 0.5 mM cobalt nitrate in 0.1 M potassium phosphate buffer at pH 7, but with 1 sun AM 1.5 simulated sunlight illumination. Because conduction-band electrons in α-Fe₂O₃ do not have sufficient potential to reduce water, an external bias (˜0.1-0.4 V) was applied. The amount of Co-Pi was again controlled by the deposition time, which ranged between 500-750 s. Current densities were typically ˜2-5 μA/cm² during deposition.

Following Example 4, Co²⁺ adsorption onto mesostructured α-Fe₂O₃ photoanodes was achieved by dipping the photoanode in a solution of 0.1 M cobalt nitrate for 5 minutes. The amount of Co²⁺ adsorbed was optimized by repetition of this dipping process. Typically, PEC enhancement reached its maximum after about three cycles. Subsequent cycles resulted in either no change or a decrease in the PEC performance.

PEC measurements were conducted in 1M NaOH (pH 13.6) using a three-electrode configuration, with the photoanode as the working electrode, Ag/AgCl as the reference electrode, and Pt as the counter electrode. Photocurrent densities were measured with front-side illumination under 1 sun AM 1.5 simulated sunlight using an Oriel 96000 solar simulator equipped with a 150 W Xenon arc lamp and an Oriel AM 1.5 filter. Potentials vs. RHE are calculated using the Nernst equation E_(RHE)=E_(Ag/AgCl)+0.0591(pH)+0.1976 V. Very similar α-Fe₂O₃ photoanodes were used for all PEC measurements. The amount of catalyst applied was optimized to give the largest sustainable cathodic shift and overall current density by controlling the amount of catalyst loading, either by adjusting the time of deposition for Co-Pi or the number of cobalt dipping cycles for Co²⁺ adsorption. Cathodic shifts were calculated as the average voltage shifts in the window where current densities range from 0.5-1.5 μA/cm². For uniformity, reported photocurrent increases with catalyst deposition refer specifically to the difference in photocurrent at 1.1 V vs RHE. Photocurrent onset potentials were calculated by extrapolation to zero current from the linear portion of the J-V curve where current densities range from 0.5-1.5 mA/cm².

Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses were performed using a FEI Sirion SEM equipped with an energy dispersive spectrometer. No conductive coating was deposited onto samples for these measurements.

SEM images of a representative mesostructured α-Fe₂O₃ photoanode are shown in FIGS. 22A and 22B. The photoanode possesses the dendritic features typical of α-Fe₂O₃ grown by APCVD. All catalyst-modified photoanodes show similar dendritic features but the images are slightly blurred (FIG. 22 C—H), suggesting that the catalysts make the surfaces more insulating and hence more susceptible to charging effects from the electron beam. All photoanode surfaces appear uniform except for the one involving electrodeposited Co-Pi (FIG. 22 C,D), which shows patches of Co-Pi. On the other films, the catalyst itself is not resolved by the SEM measurement, but it can be detected by EDX. EDX measurements on large and small areas of the films from FIG. 22 E-H yield similar results, indicating uniform cobalt coverage on these length scales (TABLE 1).

TABLE 1 Material Probe area Fe (%) Co (%) α-Fe₂O₃ 1700 μm²  38.11 — Photodeposited 430 μm² 33.35 1.00 Co-Pi/α-Fe₂O₃ Photodeposited 0.25 μm²  35.52 1.06 Co-Pi/α-Fe₂O₃ (single nodule) Electrodeposited 430 μm² 33.71 1.23 Co-Pi/α-Fe₂O₃ Co²⁺ adsorbed/α-Fe₂O₃ 1700 μm²  34.02 0.94

The amount of catalyst on α-Fe₂O₃ photoanodes that yields the largest sustainable PEC enhancement can be roughly estimated using the EDX results. As expected for surface deposition, increasing the probe depth by increasing the electron acceleration voltage from 10 to 15 keV results in a substantial decrease in the relative cobalt peak intensity. Approximating the probe depth of a 10 keV electron beam to be ˜200 nm the assumption of a uniform flat surface would yield a Co-Pi thickness of ˜30 nm, but this value represents an upper limit because of the very high surface roughness of the α-Fe₂O₃ mesostructure (roughness ˜20). The active Co-Pi cluster is believed to possess seven cobalt ions, with a volume of ˜700 Å, from which an upper limit of 34 clusters thickness is obtained. In all likelihood, the actual thickness is substantially smaller. For example, it is interesting to note that the amount of cobalt detected by EDX is about the same for optimized Co-Pi/α-Fe₂O₃ as for Co²⁺-impregnated α-Fe₂O₃. Co²⁺ adsorption has previously been suggested to yield only monolayer coverage, implying closer to one monolayer of Co-Pi cluster as well. Overall, these results clearly indicate that Co-Pi/α-Fe₂O₃ composite photoelectrodes optimized for steady-state photocurrents possess far thinner Co-Pi layers than the analogous Co-Pi-coated electrodes used in electrocatalysis. This difference relates to the kinetic bottleneck described previously, which likely reflects the important role of surface electron-hole recombination under PEC conditions.

FIG. 23A-C compares current-voltage (J-V) characteristics of representative Co-Pi/α-Fe₂O₃ and Co²⁺-modified α-Fe₂O₃ photoelectrodes. All photoelectrodes have been optimized to give the largest steady-state cathodic shift and PEC enhancement compared to their parent α-Fe₂O₃ photoanodes. Photo-assisted electrochemical deposition of Co-Pi onto α-Fe₂O₃ (FIG. 23A) yields the greatest cathodic shift of the onset potential for PEC water oxidation, ˜170 mV. Similar results were described previously for electrochemical deposition of Co-Pi onto α-Fe₂O₃ following optimization. The best electrodeposited Co-Pi/α-Fe₂O₃ photoanode in this set showed a ˜100 mV cathodic shift (FIG. 23B), and the best Co²⁺-impregnated α-Fe₂O₃ photoanode showed an ˜80 mV shift (FIG. 23C).

The current densities for each of these films are stable and reproducible after multiple J-V scans and under illumination for over 72 hours, even after weeks of storage at room temperature in air. FIG. 28 illustrates the time dependence of the photocurrent density of a Co-Pi/α-Fe2O3 photoanode prepared by photo-assisted electrochemical deposition, measured at 1.0 V vs RHE in 1 M NaOH under continuous 1 sun, AM 1.5 simulated solar irradiation. The electrolyte was not stirred. The electrolyte was replaced after 75 hrs (dashed line), resulting in recovery of photocurrent density.

Interesting variations in performance are observed from film to film, much of which derives from variations in the underlying α-Fe₂O₃ photoanode performance. Such differences are illustrated in FIG. 24A-24C, which shows the photocurrent responses of two quite different composite photoanodes in comparison with those of is their parent α-Fe₂O₃ photoanodes. The photoanode in FIG. 24A shows large, stable photocurrent densities at high bias, whereas the one in FIG. 24B excels at low bias. These differences are due to a small variation in the deposition temperature. These data also emphasize that Co-Pi surface deposition has a similar effect on each parent α-Fe₂O₃ photoanode, despite their absolute performance differences. Both films show comparable cathodic shifts of their photocurrent onset potentials and small enhancements of their maximum photocurrent densities upon deposition of Co-Pi.

Incident-photon-to-current conversion efficiency (IPCE) measurements on a Co-Pi/α-Fe₂O₃ photoanode prepared by photo-assisted electrochemical deposition (FIG. 24C) show a value of 40% at 400 nm and 1.23 V vs RHE, with large visible-light conversion efficiencies even at lower bias. The photocurrent response spectrum of the Co-Pi/α-Fe₂O₃ photoanode exhibits the same features as α-Fe₂O₃, indicating the primary photoresponse is from the α-Fe₂O₃ mesostructure and not in the Co-Pi itself. We note the particularly strong response from the indirect bandgap feature at ˜550 nm relative to many other α-Fe₂O₃ PEC cells. The prominence of this band here is attributed to the very high surface areas of these dendritic α-Fe₂O₃ photoanodes, which allow hole harvesting even following excitation of such a localized transition, and to the role of Co-Pi in facilitating productive use of those holes for water oxidation.

To put the above comparisons on a more quantitative footing, FIGS. 25A-25C summarizes the PEC results obtained from the investigation of a total of 12 catalyst-modified photoanodes, with particular care given to ensuring that they all involved very similar parent α-Fe₂O₃ photoanodes as their starting points. The maximum cathodic shift (FIG. 25A), maximum photocurrent density increase (FIG. 25B), and absolute photocurrent onset potentials (FIG. 25C) of the best photoanodes in each category are plotted as a bar graph. The average performance in each category is indicated by a horizontal line in the top two graphs and by an empty bar in the bottom graph. Plotting one metric vs another confirms the linear relationship between cathodic shift and reduced photocurrent onset potential (FIG. 26). Similarly, greater absolute photocurrent densities are strongly correlated with larger increases in photocurrent density upon Co-Pi deposition (FIG. 27). From these data, it is concluded that photo-assisted electrochemical deposition of Co-Pi onto α-Fe₂O₃ photoanodes yields both a lower onset potential and a greater increase in photocurrent density than either Co-Pi electrochemical deposition or Co²⁺ surface adsorption.

Specifically, FIG. 26 illustrates the average cathodic shifts plotted vs average onset potentials for Co-Pi/α-Fe2O3 photoanodes prepared by photo-assisted electrochemical deposition (P-Dep) and electrochemical deposition (E-dep) of Co-Pi, and for Co2+/α-Fe2O3 photoanodes prepared by surface adsorption of Co2+(Co-dip) for the films used to generate the data of FIGS. 25A and 25C. The open symbols represent the parent α-Fe2O3 photoanodes. These data show a strong correlation between the two performance metrics, with photo-assisted electrochemical deposition of Co-Pi leading to the lowest onset potentials and the greatest cathodic shifts.

FIG. 27 illustrates the Average photocurrent density increase vs photocurrent at 1.1 V vs RHE (one-sun) for the films used to generate the data of FIGS. 25B and 25C. Photo-assisted electrochemical deposition of Co-Pi yields the largest photocurrent density increases and the highest absolute photocurrent densities. The open symbols (grouped at the base of the dashed line) represent the parent α-Fe2O3 photoanodes.

Overall, these data clearly reveal the superiority of photo-assisted electrochemical deposition over simple electrochemical deposition for the preparation of Co-Pi/α-Fe₂O₃ composite photoanodes. They also illustrate the improvements in α-Fe₂O₃ PEC performance obtained using Co-Pi rather than surface-adsorbed Co²⁺ as the electrocatalyst.

In addition to their ease of preparation, Earth-abundant composition, and highly stable photocurrent densities, the absolute performances of Co-Pi/α-Fe₂O₃ photoanodes are comparable with those of IrO₂/α-Fe₂O₃ photoanodes prepared by attachment of nanocrystals of the well-known water oxidation catalyst, IrO₂, onto similar α-Fe₂O₃ photoanodes. Compared to the Co-Pi/α-Fe₂O₃ photoanode in FIG. 24A, the best IrO₂/α-Fe₂O₃ photoanode showed a 50 mV greater cathodic shift, a 60 mV lower onset potential, and a ˜13% larger photocurrent density at 1.23 V vs RHE. An important difference between Co-Pi/α-Fe₂O₃ and IrO₂/α-Fe₂O₃ photoanodes, however, is that the photocurrent responses of the IrO₂/α-Fe₂O₃ photoanodes appear to diminish on short (200 s) timescales because of detachment of the IrO₂ particles from the α-Fe₂O₃ surface. The Co-Pi/α-Fe₂O₃ composite photoanodes show no similar instability (see supporting information).

Despite the reduced onset potential, a positive voltage must still be applied in order to drive PEC water oxidation using α-Fe₂O₃. Ideally, this voltage would be supplied by a photovoltaic (PV) device in a tandem configuration. For the photoanode in FIG. 24A, the increase in photocurrent density from 2.1 to 2.8 mA/cm² at 1.23 V (the thermodynamic potential for electrolysis) following Co-Pi deposition corresponds to a 33% improvement and yields a solar to hydrogen conversion efficiency of h_(sth)=3.4%, based on the Gibbs free energy of the reaction and assuming a faradaic efficiency of unity. At 1.43 V, the photocurrent density of 3.3 mA/cm² corresponds to h_(sth)=4.1%. Unfortunately, most low cost PV devices such as DSSCs provide less than 1.0 V at open circuit, and multiple PV devices connected in series would thus be required to achieve the, above efficiencies. To minimize cost, PEC photocurrent densities at low applied potentials should be optimized, and the cathodic shifts provided by Co-Pi modification are therefore of interest. The Co-Pi/α-Fe₂O₃ photoanode of FIG. 24B shows a relatively high photocurrent density of 1.0 mA/cm² at 1.0 V vs RHE, which constitutes a 500% improvement over α-Fe₂O₃ alone at the same voltage (0.2 mA/cm²).

In summary, photo-assisted electrochemical deposition of Co-Pi onto mesostructured α-Fe₂O₃ yields better performing photoanodes than either electrochemical deposition of Co-Pi or simple Co²⁺ wet impregnation. A stable ˜170 mV cathodic shift was observed with photoelectrochemical deposition of Co-Pi, while the electrochemical deposition of Co-Pi gave cathodic shifts of ˜100 mV, and Co²⁺ impregnation gave ˜80 mV cathodic shifts. Photo-assisted electrochemical deposition provides a more uniform distribution of Co-Pi on α-Fe₂O₃ than obtained by electrochemical deposition by allowing deposition only where visible light generates oxidizing equivalents. Optimization of the photo-assisted electrochemical deposition conditions allowed elimination of all nodules and islands to yield thin uniform films of Co-Pi over the entire photoanode surface. The resulting catalyst-modified metal-oxide photoelectrodes are attractive for solar water oxidation in tandem or hybrid PEC cells.

Example 4 Deposition of Cobalt Oxide Catalysts on α-Fe₂O₃ by Deposition from an Aqueous Solution of Co²⁺, Such as from Cobalt Nitrate, Cobalt Acetate or Cobalt Sulfate

Electrochemical deposition and photoelectrochemical deposition of a cobalt oxide catalyst, referred to here as “CoO_(x),” on α-Fe₂O₃ were produced by deposition from an aqueous solution of Co²⁺, such as from cobalt nitrate, cobalt acetate or cobalt sulfate. X-ray diffraction experiments showed that CoO_(x) did not match the typical diffraction patterns of known cobalt oxides, CoO, Co₂O₃, or Co₃O₄. In one embodiment, CoO_(x) was electrodeposited from an aqueous solution of 10 mM cobalt nitrate (pH ˜4) at 0.7-1.4 V vs Ag/AgCl. A Pt mesh was used as the counter electrode and saturated Ag/AgCl was used as the reference electrode. In another embodiment, CoO_(x) was photodeposited on α-Fe₂O₃ from the same Co²⁺ electrolyte under a light bias, 1 sun AM 1.5 simulated solar irradiation, and at 0.1-0.4 V vs Ag/AgCl. FIG. 19 shows the J-V characteristics of α-Fe₂O₃ and the composite CoO_(x)/α-Fe₂O₃ photoanode after electrochemical deposition in 1 M NaOH. Dark current (dotted) and photocurrent (solid) are illustrated. A ˜100 mV cathodic shift of the onset potential for water oxidation was observed in CoO_(x)-modified α-Fe₂O₃. A similar effect was also observed in CoO_(x)/α-Fe₂O₃ composite photoanodes after photoelectrochemical deposition of CoO_(x) (FIG. 20). These data demonstrated that electrochemical deposition and photoelectrochemical deposition of catalysts on α-Fe₂O₃ are not limited to Co-Pi, but are applicable to other Co-based oxygen evolving catalysts as well.

Example 5 Composite Co-Pi/TiO₂—NW Photoanode

A photo-assisted electrochemical deposition (photoelectrochemical) approach was employed to achieve selective deposition of Co-Pi onto TiO₂ nanowires (NWs).

FIG. 29 illustrates current density-voltage curves of a TiO₂ nanowire photoanode before and after Co-Pi photoelectrochemical deposition, measured under 1 sun, AM 1.5 simulated solar irradiation (solid) and in the dark (dashed) in 0.1M potassium phosphate buffer at pH 7. Co-Pi modification results in a ˜190 mV cathodic shift in the photocurrent.

These data demonstrate that the cobalt-containing catalyst Co-Pi can be photoelectrochemically deposited onto other semiconductor materials of different shapes, such as TiO₂ nanowires, as well as onto dendritic α-Fe₂O₃ photoanodes. Simple electrodeposition of Co-Pi onto the same TiO₂ nanowire structures grown on conductive FTO substrates results in preferential catalyst deposition onto the exposed more-conductive FTO instead and does not improve PEC water oxidation performance. Direct photodeposition of Co-Pi did not result in successful application of the catalyst. By photo-assisted electrodeposition using a wavelength at which only the TiO₂ absorbs, Co-Pi was successfully applied specifically to the TiO₂ nanowires, yielding the significant cathodic shift in the PEC water oxidation potential. This result also shows that Co-Pi can be used to improve the PEC water oxidation of a semiconductor such as TiO₂ with an already low onset potential towards PEC water oxidation. The successful Co-Pi modification of TiO₂ nanowires demonstrates the versatility of this photoelectrochemical deposition method to apply cobalt-containing water oxidation catalysts onto semiconductor materials of various shapes and sizes.

Example 6 Composite Co-Pi/Amorphous TiO2/CdS/TiO2 NW Photoanodes

FIG. 30 illustrates current density-voltage curves of a TiO₂ nanowire photoanode sensitized with CdS nanoparticles coated with a thin amorphous TiO₂ protective layer, before and after Co-Pi photoelectrochemical deposition, measured under 1 sun, AM 1.5 simulated solar irradiation in 0.5M sodium thiosulfate.

These data demonstrate that the catalyst Co-Pi can be deposited by photoelectrochemical deposition onto complex electrodes involving visible-light-absorbing sensitizers, such as CdS, integrated with UV light absorbing wide-bandgap semiconductors, such as TiO₂, via photoexcitation of the sensitizer and an applied potential. By modifying the TiO₂ nanowire surfaces with CdS (bandgap 2.4 eV), the PEC water oxidation electrode is made more sensitive to visible light (i.e., sunlight), as seen by the large photocurrent enhancement. A cathodic shift is also observed after catalyst modification, demonstrating the compatibility of this catalyst deposition method with sensitizers such as CdS and with complex electrodes involving both sensitizers and wide-gap oxides.

Example 7 Composite Co-Pi/Co²⁺:ZnO Photoanodes

FIG. 31 illustrates current density-voltage curves of a Co²⁺:ZnO photoanode before and after Co-Pi photoelectrochemical deposition, measured under 1 sun, AM 1.5 simulated solar irradiation (solid) and in the dark (dotted) in 0.1M potassium phosphate buffer at pH 11.

These data demonstrate that photoelectrochemical deposition can also be applied to deposited Co-Pi onto wide-gap semiconductors doped with cationic impurities (Co²⁺) introduced to extend PEC water oxidation into the visible region and increase the solar photocurrent densities relative to undoped ZnO. Photoelectrochemical deposition of catalysts onto such doped semiconductors can also be achieved via excitation of mid-gap electronic transitions arising from the dopants, demonstrating that the photoelectrochemical deposition method is not limited to bandgap excitation of semiconductors. Regardless of the electronic transition used for photoelectrochemical deposition, the result is an increase in the overall PEC water oxidation efficiency.

Example 8 Composite Co-Pi/W:BiVO₄ Photoanodes

FIG. 32 illustrates current density-voltage curves of a W-doped BiVO₄ photoanode before and after Co-Pi photoelectrochemical deposition, measured under 1 sun, AM 1.5 simulated solar irradiation (solid) and in the dark (dashed).

These data demonstrate that photoelectrochemical deposition of Co-Pi can also be used to deposition water-oxidation catalysts onto doped ternary semiconductors, such as W:BiVO₄. The result is a substantial >300 mV cathodic shift in the onset potential for PEC water oxidation and a significantly lower onset potential (<400 mV vs RHE) than can be achieved with Co-Pi/α-Fe₂O₃ composite photoanodes. As with α-Fe₂O₃, a thin uniform layer of catalyst is desired for large stable photocurrent improvements. Increased deposition of Co-Pi onto W:BiVO₄ results in decreased PEC performance associated with thick catalyst layers. Photoelectrochemical deposition is a useful approach for applying thin well-dispersed catalyst layers onto various types of semiconductor photoanodes.

Example 9 Composite Cobalt Methyl-Phosphonate/α-Fe₂O₃ Photoanodes

FIG. 33 illustrates current density-voltage curves of an α-Fe₂O₃ photoanode before and after cobalt methyl-phosphonate (Co-MePi) photoelectrodeposition, measured under 1 sun, AM 1.5 simulated solar irradiation (solid) and in the dark (dotted) in 1 M NaOH, pH 13.6.

These data demonstrated that other cobalt-containing water-oxidation electrocatalysts besides Co-Pi, such as Co-MePi, can be successfully photoelectrochemically deposited onto a semiconductor, such as α-Fe₂O₃ to improve electrode performance. The resulting cathodic shift in the onset potential for PEC water oxidation is very similar to that achieved by Co-Pi deposition, indicating that the photoelectrodeposition method can be expanded to include other oxygen evolving electrocatalysts as well.

Example 10 Composite Nickel Borate/α-Fe₂O₃ Photoanodes

FIG. 34 illustrates current density-voltage curves of an α-Fe₂O₃ photoanode before and after nickel borate (Ni—Bi) electrodeposition, measured under back-side illumination with 1 sun, AM 1.5 simulated solar irradiation (solid) and in the dark (dotted) in 1 M NaOH, pH 13.6.

These data demonstrate that other electrocatalysts, such as the nickel-based Ni—Bi catalyst, can be applied onto semiconductors, such as α-Fe₂O₃, by photoelectrochemical deposition to yield a favorable cathodic shift of the onset potential for PEC water oxidation. This illustration indicates that photoelectrochemical deposition is a general approach for the application of various oxygen evolving electrocatalysts onto a variety of semiconductor photoanodes.

While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention. 

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A method of forming a composite photoanode, comprising photoelectrodepositing a solid conformal layer of an electrocatalyst from an electrolyte solution on a surface of a semiconductor submerged in the electrolyte solution by simultaneously: (1) impinging the surface of the semiconductor with electromagnetic radiation having a first wavelength and a first irradiance, to provide a first photoenergy that is sufficient to excite an electronic transition of the semiconductor; and (2) applying a first electric bias to the semiconductor, wherein the first electric bias is less than an electrochemical deposition bias, said electrochemical deposition bias being the minimum voltage required to electrodeposit the electrocatalyst onto the surface of the semiconductor without impinging the surface of the semiconductor with electromagnetic radiation having the first photoenergy; wherein the combination of the first photoenergy and the first electric bias are sufficient to deposit catalyst components from the electrolyte to form the solid conformal layer of the electrocatalyst.
 2. The method of claim 1, wherein the electronic transition is a bandgap transition.
 3. The method of claim 1, wherein the semiconductor has a physical shape selected from the group consisting of dendrites, wires, belts, rods, mesostructures, nanotubes, and thin films.
 4. The method of claim 3, wherein said physical shape has nanoscopic dimensions.
 5. The method of claim 1, wherein the semiconductor comprises hematite iron oxide dendrites.
 6. The method of claim 1, wherein the electrocatalyst is selected from the group consisting of a cobalt-containing catalyst, an iridium-containing catalyst, a manganese-containing catalyst, a ruthenium-containing catalyst, a nickel-containing catalyst, a cobalt-containing oxygen evolving catalyst, a cobalt oxide/hydroxide catalyst, and a cobalt oxide catalyst.
 7. The method of claim 1, wherein the electrocatalyst is cobalt phosphate.
 8. The method of claim 1, wherein the layer of the electrocatalyst has a thickness of from 0.5 nm to 30 nm.
 9. The method of claim 1, wherein the semiconductor is an n-type semiconductor.
 10. The method of claim 1, wherein the formed photoanode, when incorporated into a photoelectrochemical cell for electrolysis of water into oxygen, reduces a water electrolysis onset voltage compared to a second photoanode comprising the semiconductor without the electrocatalyst.
 11. The method of claim 10, wherein the water electrolysis onset voltage is reduced by 50 mV to 400 mV.
 12. The method of claim 1, wherein the first wavelength of the electromagnetic radiation is from 300 nm to 800 nm.
 13. The method of claim 1, wherein the first irradiance of the electromagnetic radiation is from 0.1 W/m² to 1100 W/m², or the equivalent in pulsed irradiation.
 14. The method of claim 1, wherein the electromagnetic radiation is selected from the group consisting of continuous radiation and pulsed radiation.
 15. The method of claim 1, wherein the first electric bias is applied to the semiconductor as part of an electrochemical deposition system comprising a power source in electrical communication with the semiconductor and a counter electrode.
 16. The method of claim 1, wherein the electrolyte solution comprises cations selected from the group consisting of cobalt, iridium, manganese, nickel, and ruthenium.
 17. The method of claim 1, wherein the electrolyte solution comprises anions selected from the group consisting of phosphate, methyl phosphonate, borate, acetate, sulfate and hydroxide.
 18. The method of claim 1, wherein the semiconductor comprises a sensitizer having a sensitizer absorbance wavelength, said sensitizer absorbance being different from a semiconductor absorbance wavelength.
 19. The method of claim 1, wherein the semiconductor comprises a group IV semiconductor having a formula selected from the group consisting of binary, ternary, and quaternary.
 20. The method of claim 19, wherein the group IV semiconductor further comprises ions selected from the group consisting of cations and anions.
 21. The method of claim 1, wherein the semiconductor comprises a material selected from the group consisting of an iron oxide, a zinc oxide, a titanium oxide, a tungsten-bismuth-vanadium oxide, a tungsten oxide, a gallium-zinc-oxide-nitride, or these materials also containing additional cations or anions.
 22. The method of claim 1, the wherein the combination of the first photoenergy and the first electric bias are sufficient to oxidize cations to deposit catalyst components from the electrolyte to form the solid conformal layer of the electrocatalyst. 